ORP electrodes measure the voltage across a circuit formed by the measuring metal half cell and the reference half cell. When the ORP electrode is placed in the presence of oxidizing or reducing agents, electrons are constantly transferred back and forth on its measuring surface, generating a small amount of voltage.
Oxidation is the loss of electrons by an atom, molecule, or ion. It may or may not be accompanied by the addition of oxygen, which is the origin of the term.
Familiar examples are iron rusting and wood burning. When a substance has been oxidized, its oxidation state increases. Many substances can exist in a number of oxidation states. A good example is sulfur, which can exhibit oxidation states of
-2 (H2S); 0 (S); +4 (SO2); and +6 (SO4 -2).
Substances with multiple oxidation states can be sequentially oxidized from one
oxidation state to the next higher. Adjacent oxidation states of a particular substance are referred to as redox couples. In the case below, the redox couple is
Fe = Fe+2 + 2e-iron ferrous ion electrons
The chemical equation shown above is called the half-reaction for the oxidation, because, as will be seen, the electrons lost by the iron atom cannot exist in solution and have to be accepted by another substance in solution. So the complete reaction involving the oxidation of iron will have to include another substance, which will be reduced. The oxidation reaction shown for iron is, therefore, only half of the total reaction that takes place.
Reduction is the net gain of electrons by an atom, molecule, or ion. When a chemical substance is reduced, its oxidation state is lowered. As was the case with oxidation, substances that can exhibit multiple oxidation states can also be sequentially reduced from one oxidation state to the next lower oxidation state. The chemical equation shown below is the half-reaction for the reduction of chlorine:
Cl2 + 2e- = 2Clchlorine
electrons chloride ion The redox couple in the above case is Cl2/Cl- (chlorine/
chloride). Oxidation reactions are always accompanied by reduction reactions. The electrons lost in oxidation must have another substance as a destination, and the electrons gained in reduction reactions have to come from a source. When two half-reactions are combined to give the overall reaction, the electrons lost in the oxidation reaction must equal the electrons gained in the reduction reaction.
OXIDATION: Fe = Fe+2 + 2 e-(Half- Reaction)
REDUCTION: Cl2 + 2 e- = 2 Cl- (Half- Reaction)
OVERALL REACTION: Fe + Cl2 => FeCl2
In the reaction above, iron (Fe) reduces chlorine (Cl2) and is called a reductantor reducing agent. Conversely, chlorine (Cl2) oxidizes iron (Fe) and is called an oxidantor oxidizing agent: